This can only verify that your CGI backend (CGI,
Catalyst,
CGI::Simple) thinks it was a file upload.
If the user submits a filename which doesn't exist on their system,
you will probably get a valid HTML::FormFu::Upload object,
with a valid filehandle,
but no Content-Length.
This Constraint would still see this as a valid uploaded file - if you want to ensure that you get a file with content,
instead use HTML::FormFu::Constraint::File::Size with min set to 1.